It is estimated that 170 million people worldwide are infected with the Hepatitis C virus (HCV) and are at risk of developing chronic hepatitis or cirrhosis, the latter often leading to hepatocellular carcinoma. There is currently no vaccine and licensed therapies are associated with modest efficacies and significant toxicities. Despite the urgency of this worldwide public health problem, our basic understanding of HCV replication and pathogenesis remains poor due to a lack of key experimental models. For example, difficulties in culturing the virus in vitro and expressing native, fusogenic envelope glycoproteins have greatly limited studies of HCV tropism and entry. These are critical aspects of viral biology because the host range and pathogenesis of enveloped virus infection is largely determined by the selective interaction of viral envelope glycoproteins with cell-surface receptors. A major goal in HCV research is to understand how HCV targets the liver and by what mechanism it enters host cells. Recently, a major breakthrough in the field has been the development of retroviruses pseudotyped with HCV envelope glycoproteins that specifically mediate infection of primary hepatocytes, as well as certain other human cells. We will use this new experimental system to study HCV entry into target cells. Alterations in naturally occurring HCV envelope glycoproteins may predicate differences in receptor usage and target cell tropism in vivo. To investigate the range of HCV cellular tropism, pseudotypes incorporating envelope glycoproteins from clinical HCV isolates will be tested for their ability to enter relevant primary cells and cell lines. A functional cDNA cloning approach will be used to identify cell-surface receptors that specifically mediate HCV entry into different target cells. However, these receptors may be ubiquitously expressed and HCV targeting to different cell types may be determined by another mechanism. We recently demonstrated that L-SIGN and DC-SIGN are specific HCV-capture receptors and we will explore whether they mediate infection of target cells in trans, thereby determining HCV tropism. The major objective of our work is to identify the basic protein interactions that mediate HCV tropism, which will serve as a foundation for detailed structure/function analyses of HCV receptors and envelope glycoproteins. [unreadable] [unreadable] [unreadable] [unreadable]